Mixing and Combustion Performance of a Stratified Bluff Body Primary Zone Interacting with a Coannular Swirl–Induced Recirculation

Abstract

The interaction of an axisymmetric, bluff body-stabilized, primary zone, operated under stratified inlet mixture conditions, with a coannular, secondary swirling stream and an external, surrounding air coflow was investigated. The coannular assembly establishes an axial sequence of two recirculations, the bluff body zone and the adjacent swirl-induced vortex breakdown region that promotes mixing of the combustion products with the swirl stream. The rate and efficiency of admixing of primary, secondary, and external stream gases and the entrainment behavior of the twin vortex system was studied for inert conditions, under different inlet settings and combinations of fuel injection placement in either the primary or swirl stream. The counterpart lean and ultralean reacting wakes were then studied to appraise the capacity of the system to regulate effectively the primary combustion process. Measurements of fuel-air mixing concentrations, temperatures, chemiluminescence imaging of OH* and CH*, and gas analysis assisted in this preliminary evaluation of the variations in flame structure, mixing topology, and combustion performance. Complementary computations of the mixing fields were performed to provide insight into the flow patterns that support flame stabilization. The differences and similarities between the present flame stabilizing configuration and other types of axisymmetric arrangements are also highlighted and discussed.